The U.S. Attorney's Office in Utah is not aware of an active federal investigation into the disappearance of Susan Cox Powell, according to the Associated Press.

Anne Bremner, an attorney for the Cox family, told reporters Tuesday that federal officials were looking into what the missing woman's father-in-law, Steven Powell, may have known.

"And we've confirmed that this morning and so we're very heartened by that," Bremner said at her office in downtown Seattle. "… there will be further investigation into this case involving Susan Cox Powell's disappearance and other circumstances we know about."

Bremner's announcement came a day after police in West Valley City, Utah, where Susan was living when she vanished in 2009, said their active investigation had come to an end.

The Salt Lake City U.S. Attorney's Office told AP that authorities would be happy to help if new developments emerge, but are not actively working on the case. The FBI in Salt Lake City would not comment.

Bremner said she could not provide specifics about the investigation she had been briefed on.

The West Valley City Police Department Monday released more than 30,000 pages of documents on the case Monday. In them, authorities discussed details of a possible affair that Josh Powell had before his wife went missing. Police also said they suspected Josh and his brother Michael played a significant role in her disappearance.

Josh Powell had replaced Susan with his brother as beneficiary on a $3.5 million life insurance policy shortly after her disappearance. Josh killed himself and his two young sons and then left all of his belongings to Michael, who later jumped to his death from a parking garage in Minnesota.

His suicide left investigators without any person of interest in the case. While authorities believe the brothers were responsible for Susan Powell's disappearance, they said repeatedly Monday that they never had enough evidence to bring charges - an assertion that has been questioned in the past by legal experts as well as law enforcement in Washington state.

"I think there were a lot of secrets kept in this case and there are a lot of answers we want to find," said Bremner.

Steven Powell is eligible for release from a Washington state prison this Thursday. He was convicted on multiple counts of voyeurism for taking photos of young girls in his Puyallup neighborhood without their knowledge.

Susan Powell's father Chuck Cox said he's worried Powell remains a danger to the community.

The Cox family has hired a private investigator to help them continue their search. And even though they admit they believe Susan is dead, they plan to continue working with the various foundations they've created in the name of Susan and her two sons Charlie and Braden to combat domestic violence, help foster children and other causes.

"My daughter is gone. She's not coming back," Cox said. "If it raises the attention of people and stops it from happening to other people then that's a positive thing to come out of this."